People with diabetes typically measure their blood glucose level by lancing a finger tip or other body location to draw blood, applying the blood to a disposable test strip in a hand-held meter and allowing the meter and strip to perform an electrochemical test of the blood to determine the current glucose concentration. Such in vitro tests are typically conducted at least several times per day. Detailed descriptions of such glucose monitoring systems and their use are provided in U.S. Pat. No. 7,058,437, issued to TheraSense, Inc. on Jun. 6, 2006, which is incorporated by reference herein in its entirety.
In addition to the examples provided in U.S. Pat. No. 7,058,437, there have been numerous other approaches to test strip sensor construction in the field of in vitro blood glucose monitoring. Two common methods are described below.
In the first common method of test strip construction, a mesh, insulation and lidding tape arrangement is used. In this method, a base electrode is first formed on a substrate. A surfactant-coated mesh is then adhered to the base electrode by an overprinted layer of insulation ink. The ink is applied in a printed pattern. The open (non-printed) area of the pattern forms the sample cell and defines the working area on the base electrode. A lidding tape is then adhered to the upper surface of the insulation printing leaving sufficient openings for the air to escape as the strip fills with blood during use.
A disadvantage of this method is that print registration accuracy and ink rheology limits the smallest size of cell that can be manufactured repeatably. In addition, three separate processing steps are required, and the mesh and insulation materials are relatively expensive.
In the second common method of test strip construction, a die-cut spacer and hydrophilic lidding tape are used. This method typically involves laminating a die-cut spacer to a hydrophilic lidding tape. The lidding tape is in turn laminated to a base electrode on a substrate. In most cases, the adhesive used at all of the interfaces is pressure sensitive. The thickness of the spacer and layers of adhesive, coupled with the two-dimensional area removed from the spacer, define the volume of the sample cell.
A disadvantage of this second method is that gumming problems are often encountered when cutting pressure sensitive adhesives. Test strip manufacturing equipment is typically what gums up, but test strip ports on a user's meter can also be disabled by fouling caused by the adhesives. Additionally, mechanical punches can only be scaled down to a certain size. Also, in this process registration is critical in all planes, and the materials used are expensive.
Applying a reagent coating during the manufacture of test strips presents a challenge in situations where pad printing is not suitable. This challenge has been solved in two different ways, slot coating and spraying, each of which is described in turn below.
Slot coating uses a slot die and reagent pump to dose material onto a moving web. The pump rates, web transport rates, reagent rheology and slot geometry are all critical factors in achieving the desired coating. This method can be an ideal way of applying low viscosity reagents in a controlled manner at high speeds. However, it suffers from a number of problems. The first problem is that it is a continuous process and therefore coats areas of the web that are not functionally required for the assay. This not only is wasteful of reagent but also causes variations in height on the sides of the sample chamber, creating problems when sealing the chamber. If the sample chamber is not well sealed, the sample blood may leach away from the defined measurement area and provide erroneous results. Finally, providing a uniform stripe with the slot coating method can be problematic with some liquids since thicker bands of material are often found at the edges of the coated stripe.
Spraying is another method of laying fine coatings of reagent onto moving webs but also suffers from some disadvantages. In a reversal of the situation seen with slot coating, it is not uncommon for the center of the stripe to be thicker than the edges. This helps with sample chamber sealing, but is also not uniform. Since spraying is also a continuous process, it too is wasteful of reagent, and it is difficult to define areas accurately without masks.
Even with tight controls on strip manufacture, there typically is variation across different strip lots. In order to maintain accurate test results, some type of strip calibration is usually employed. For example, a representative sample of strips from each lot can be tested after manufacture. A calibration code can be determined from the testing and this code can be provided with each strip in the associated lot, such as on a packaging label. Before use of each package of test strips, the code can be entered into the meter, thereby calibrating the meter with the particular strips being used to provide accurate test results. However, this requires the user to perform an extra step. Furthermore, if the user neglects to enter a new calibration code for a new package of strips or enters the code incorrectly, inaccurate test results may be obtained, potentially causing harm to the user. Some manufacturers have resorted to providing a machine readable code on each strip or strip packaging that can be read directly by the meter during use. While this may reduce errors, these systems are not foolproof and add cost to the test strips and meters. Another method of reducing calibration issues is to supply a sub-set of test strip production, having a given calibration code, to a given customer base having meters that are already calibrated for use with those particular test strips. The remainder of the test strip production is labeled with calibration codes and supplied to a different customer base having meters requiring manual entry of calibration codes. This method is only effective for the portion of customers that do not need to use the calibration codes. Furthermore, product supply problems can develop if the calibration distribution does not match the demand of both meter bases.